Monitoring toxic gases is a great concern in relation to environmental pollution, occupational health, and industrial emission control. Known methods and apparatuses have been developed to detect the presence of gases. For example, gas chromatography, ion chromatography, electrolytic conductivity detection, and conductometric measurement are typically used to detect gases. However, these manners for detecting gases have generally been expensive, cumbersome, and shown to have low sensitivities and slower response times.
Electrochemical sensors were provided to overcome these limitations. Electrochemical sensors typically operate at room temperature, provide signals which vary linearly with concentrations of analyte species, have improved response times, and exhibit acceptable sensitivity with high durability. In addition, electrochemical sensors are compact and can be used for continuous monitoring.
Electrochemical gas sensors usually detect the presence of gases with sufficient reliability and accuracy. However, if the humidity of the sample gas to be measured within the sensor is different than the humidity of the atmosphere surrounding the sensor, which is typically used to determine the baseline of the measurement, a sensor's accuracy may be compromised. The greater the difference in humidity, the less likely the sensor will accurately detect a gas.
U.S. Pat. No. 5,716,506 to Maclay et al. (“Maclay”) discloses an electrochemical gas sensor that detects the presence of a gas while compensating for its dependence on the humidity in the gas. Maclay discloses an invention comprising a reference sensor and an active sensor. The reference sensor utilizes electrodes that are inert with respect to the gas being detected. Hence, the reference sensor is sensitive only to relative humidity and temperature of the surrounding air. The active sensor measures the gas and humidity while taking into account the temperature. The sensor response, which depends on relative humidity, is then compensated by subtracting the response of the reference sensor from the response of the active sensor.
A disadvantage of Maclay is that it does not compensate for the relative humidity difference between the sample gas and surrounding atmosphere. Moreover, no where does Maclay disclose compensating for relative humidity by varying the thicknesses of electrolytic material on the sensing electrodes of the active and reference sensors. In fact, the compensation of the relative humidity proposed by Maclay does not effectively compensate for the difference in humidity of the sample gas and the atmosphere, where the baseline measurement is performed. Moreover, Maclay does not disclose a manner for wetting the electrolytic material where it is known in the trade that sensor response is typically dependent upon humidity.
What is desired, therefore, is an electrochemical gas sensor which compensates for relative humidity differences between sample gas and the surrounding air. What is also desired is a sensor that compensates for relative humidity without directly measuring humidity. What is further desired is an electrochemical sensor that sufficiently hydrates the electrolytic material.